Reconfigurable logic circuits such as field programmable gate arrays (FPGAs) are often used in modern electronic systems. These typically use flash memory cells to store the configuration information. A flash memory is a type of FET memory which typically has a lattice structure composed of columns and rows, a memory cell being arranged at each crossover point between the columns and rows. The memory cells have a control gate and a floating gate, separated from one another by a thin tunnel oxide layer. If an electric field is applied between source and drain, and also to the control gate, electrons can tunnel between the active region of the semiconductor substrate and the floating gate, whereby the threshold voltage of the memory cell can be switched between two states.
Although flash memory technology has undergone rapid further development in recent years, the disadvantages of long write/erase times, which typically lie in the milliseconds range, and high write/erase voltages, which typically lie in the range of 10 to 18 V, and accordingly require a large programming energy, which is in turn an obstacle to the desire for further miniaturization, have not been able to be solved heretofore. Furthermore, the fabrication method for flash memory cells is relatively complicated and thus comparatively expensive.
One approach for fabricating nonvolatile memory cells is based on the use of solid electrolytes as an active (switching) material for nonvolatile memory cells. In this case, chalcogenides, in particular, have been investigated with regard to their suitability as an active material. In this respect, see for example M. N. Kozicki, M. Yun, L. Hilt, A. Singh, Electrochemical Society Proceedings, Vol. 99-13, 298, 1999; M. N. Kozicki, M. Yun, S. J. Yang, J. P. Aberouette, J. P. Bird, Superlattices and Microstructures, Volume 27, No. 5/6, 485-488, 2000; M. N. Kozicki, et al., “Nanoscale phase separation in Ag—Ge—Se glasses”, Microelectron. Eng. 63, 155/2002; M. N. Kozicki, M. Mitkova, J. Zhu, M. Park, C. Gopalan, “Can Solid State Electrochemistry Eliminate the Memory Scaling Quandry”, Proceedings VLSI, 2002; R. Neale, “Micron to look again at non-volatile amorphous memory”, Electronic Engineering Design, 2002.
In this case, it has been illustrated, in particular, that chalcogenides, i.e., alloys containing chalcogens (elements of main group VI of the periodic table), in a solid electrolyte memory cell as has been described by Kozicki et al., for example, have good switching properties.
Solid electrolyte memory cells are based on an electrochemical redox process in which metal ions of one electrode can diffuse reversibly into and out of the solid electrolyte material and thus form and respectively resolve a low-impedance path. More precisely, a solid electrolyte is embedded between two electrodes, one electrode being formed as an inert electrode and the other electrode being formed as a reactive electrode, the reactive electrode forming together with the solid electrolyte a redox system in which a redox reaction proceeds above a defined threshold voltage (Vth). Depending on the polarity of the voltage applied to the two electrodes, which, however, must be greater than the threshold voltage, the redox reaction can proceed in one reaction direction or the other, metal ions being produced or annihilated. If, in concrete terms, an anodic potential above the threshold voltage is applied to the reactive electrode, then metal ions are produced and emitted into the solid electrolyte. Said metal ions are subsequently reduced in the solid electrolyte and form metallic precipitates. If this process is continued and if metal ions are continuously emitted into the solid electrolyte, then the metallic precipitates increase in number and/or size until a low-impedance current path bridging the two electrodes finally forms. In this state, the electrical resistance of the solid electrolyte is reduced significantly, for example by several orders or magnitude, compared with the state without a low-impedance current path, whereby the ON state of the memory cell is defined. If a voltage of opposite polarity is applied to the two electrodes, then this leads to the interruption of the low-impedance current path, which has the effect that the latter no longer electrically connects the two electrodes to one another throughout, whereby the OFF state of the memory cell is defined.
In the case of solid electrolyte memory cells, but in particular if chalcogenides are used as the solid electrolyte, the problem may arise, however, that the latter already have a sufficiently good ionic conductivity at room temperature, so that the electrochemical redox mechanism may proceed, even without external action, on account of a latent diffusion of metal ions or metal atoms that is present even at room temperature. This phenomenon leads to serious reliability problems, however, since there is the risk of a low-impedance ON state gradually undergoing transition to a high-impedance OFF state, or vice versa, without external action. Furthermore, in this case, in particular, the disadvantageous effect may occur that an unintentional rewriting of adjacent memory cells takes place on account of unavoidable capacitive couplings of adjacent memory cells. This problem occurs primarily in high-density integrated circuits. This problem area is particularly serious at comparatively high write/erase currents, but the latter may be desirable with regard to fast switching operations.
These problems have not yet been able to be solved heretofore. Thus, it has already been attempted to produce a stabler memory state by reducing the ionic mobility of the solid electrolyte at room temperature. However, the programming of such a memory cell then generally requires a brief heating of the solid electrolyte in order thereby to increase the ionic mobility, which can be achieved by applying a short external voltage pulse in order to generate Joule heat. For this purpose, use is made in practice of a solid electrolyte having a negative resistance/temperature characteristic (negative temperature coefficient), that is to say a material which generally has a sigmoidal (S-shaped) current/voltage characteristic, that is to say a current/voltage characteristic that switches between a high and a low electrical resistance.
Although a current/voltage characteristic that is sigmoidal (S-shaped) in the first quadrant has been demonstrated on various switching materials heretofore, this characteristic is generally run through reversibly, i.e., it has not been possible to demonstrate a non-volatile switching effect on these samples heretofore. In this respect, see in particular N. Fuschillo et al., “High-field transport in NiO and Ni1-xLixO thin films”, Sol. State Electron. 19 (1976) 209-216; K. C. Park, S. Basavaiah, “Bistable switching in Zr—ZrO2—Au junctions”, J. Non Cryst. Sol. 2 (1970) 284-291; J. F. Gibbons, W. E. Beadle, “Switching properties of thin NiO films”, Sol. State Electron. 7, (1964), 785-797.
Moreover, in the case of a switching element which has such an I-U characteristic that is nonvolatile, on account of the semiconducting conductivity/temperature characteristic (negative temperature coefficient) of the switching solid electrolyte (e.g., Ni—O, Zr—O compounds), the switching operation is destructive after just a few switching cycles, i.e., the solid electrolyte to be switched incurs irreversible degradation effects as a result of an avalanchelike local Joule overheating.
The US Patent Application U.S. 2003/0053350 A1 describes a solid electrolyte switching element. However, no indications about the temperature behavior of the solid electrolyte used can be gathered from this document.